The invention relates to polymeric compositions having controllable, reproducible and stable electrical conductivities of about 10.sup.-9 to 10.sup.-10 S or (ohm-cm).sup.-1. Such compositions may be used in electrophotographic image development systems such as liquid image development systems or scavengeless and hybrid scavengeless development systems. The scavengeless development systems do not scavenge or interact with a previously toned image (so as to affect image quality) and are important in trilevel and highlight color xerography (e.g. U.S. Pat. No. 4,078,929).
Two-phase conductive compositions have contained dispersions of conductive particles (e.g., carbon black or graphite) in insulating polymer matrices (e.g., dielectric binders such as a phenolic resin or fluoropolymer) close to the percolation threshold concentration. Such concentration levels allow conductive particle contact, resulting in a burst of conductivity. (See e.g., Brewington et al., U.S. Pat. No. 4,505,573.) The dielectric constant of these overcoatings ranges from about 3 to about 5, and preferably is about 3. The desired conductivity (measured in "(ohm-cm).sup.-1 " or "S") is achieved by controlling the loading of the conductive particles. However, the low conductivity values required for electrophotographic image development systems and the large, intrinsic electrical conductivity of carbon black make it extremely difficult to achieve predictable and reproducible conductivity values. Very small changes in the loading of conductive particles near the percolation threshold can cause dramatic changes in the coating's conductivity. Furthermore, differences in particle size and shape can cause wide variations in conductivity at even a constant weight loading. Moreover, the percolation threshold approach requires relatively high conductive particle concentrations. At these concentrations, the coating becomes brittle, its mechanical properties becoming controlled by carbon black rather than the polymer matrix.
Another approach is to molecularly dope an inert polymer matrix with mixtures of a neutral transport molecule and its radical cation or anion. "Molecular doping" refers to the relatively low amounts of dopant added (as compared to carbon black dispersions) to increase the polymer matrix's conductivity, and to the fact that the resulting mixture is essentially a solid solution. No chemical bonding occurs between the dopant and the charge-transporting polymer so as to produce a new material or alloy. That is, the charge-transporting polymer is rendered highly and stably conductive by molecular doping with dopants such as oxidizing agents. In the presence of an oxidizing dopant, the partially oxidized charge-transporting moieties in the charge-transporting polymer act as hole carrier sites, which transport positive charges or "holes" through the unoxidized charge-transporting moieties.
For example, Mort et al., J. Electronic Materials 9:41 (1980), discloses the possibility of chemically controlling dark conductivity by co-doping a polycarbonate with neutral and oxidized species of the same molecule, tri-p-tolylamine (i.e., TTA and TTA+). J. M. Lupinski et al., J. Polymer Science C 16:1561 (1967), discusses electrically conductive polymers consisting of a polycation and neutral and anionic 7,7,8,8-tetracyanoquinodimethane.
Limburg et al., U.S. Pat. No. 4,338,222, discloses an electrically conducting, three-component composition comprising: a polycarbonate matrix; an organic hole transport compound (particularly tetraaryl diamines); and the reaction product of the organic hole transport compound and an oxidizing agent capable of accepting one electron from the hole transport compound.
Hays et al., U.S. Pat. No. 5,300,339, discloses an overcoating comprising at least three constituents: a charge transport compound (particularly an aryl diamine), a polymer binder (particularly a polycarbonate or a polyethercarbonate), and an oxidizing agent.
None of the preceding references teaches conjugated polymers such as controllably conductive polymer compositions comprising thiophene- or oligothiophene-containing polymers; or inert elastomeric polymers, such as isoprene- or butadiene-based copolymer elastomers or polyurethane elastomers.